As CMOS devices continue to decrease in size, the need for smaller gate oxides increases, while the need for a relatively high overall oxide dielectric constant remains. Gate oxides typically consist of a combination of a relatively high k (dielectric constant) dielectric and a relatively moderate k dielectric to produce an overall oxide dielectric constant that is somewhere in between the two. Furthermore, the use of typical oxides, such as zirconium-dioxide (“ZrO2”) and hafnium-dioxide (“HfO2”), by themselves is generally undesirable, because volumetric expansion from thermal anneal cycles in semiconductor processing can result in the formation of defects in the oxide, causing leakage and reliability problems in the transistor.
Therefore, additives, such as aluminum-trioxide (“Al2O3”), are typically combined with the oxide to help it remain amorphous during exposure to high temperatures in processing. The combination of additives, such as Al2O3, and typical oxides, such as ZrO2 and HfO2, however, can result in an overall effective dielectric constant (k) that is lower than necessary to accommodate thinner oxides (<1 nm) required in modern CMOS processes. Furthermore, additives, such as Al2O3, can possess fixed charge problems as a result of the bonding configuration between the additive and the oxide.
Typical gate oxides in modern CMOS processes require a dielectric constant of at least twenty in order to support a dielectric thickness of 1 nm or less reliably. Furthermore, gate oxides must be able to withstand deteriorating effects, such as oxygen-deficient defects and thermal instability, caused by exposure to high temperatures during processing.